Single vision spectacle lens

ABSTRACT

An improved single lens comprising: 
     a continuous change in lens curvature going downwards; 
     a front and a back surfaces; 
     an upper part which front and back surfaces are top zones of front and back surfaces, covering at least the half of respectively front and back surfaces, and where the two top zones are aligned to each other such that the effective power of the upper part of the lens is constant; 
     a lower part used for viewing near objects which front and back surfaces are lower zones of front and back surfaces, the front and back lower zones comprising each a surface zone where top to bottom increasing curvature of each surface zone is provided and where the two front and back lower zones are aligned to each other such that the maximum difference of the effective power of the lower part of the lens and the effective power of the upper part of the lens is comprised between 0 and 0.5 Diopter.

The invention relates to an improved single vision spectacle lens.

Single vision lenses are prescribed when the patient is eitherfarsighted or nearsighted and have the same focal power throughout (topto bottom).

Vision correction for myopia (nearsightedness) and hypermetropia(farsightedness) can be accomplished using spectacle lenses havingappropriate spherical curves on the anterior (outer or front surface)and posterior (inner or back surface on the eye side) surfaces.Astigmatism can also be corrected when using toroidal orspherocylindrical surfaces.

Wearers of single vision lenses who are non-presbyopic have adequateaccommodation to be able to bring to focus near objects when providedwith the required correction of refractive error. However, the retinalimages of near objects are too small in size to provide adequate visualcomfort or contrast sensitivity especially at low or controlledillumination conditions, for example when reading a menu in arestaurant, or threading a needle indoors. The size of the focusedretinal image is usually expressed in the form of angular magnificationor paraxial magnification, in which magnification is defined as theratio of the retinal image size formed by a particular lens to the imagesize formed by an emmetropic eye, i.e., an eye requiring no refractivecorrection. Image magnification may also be defined for one lensconfiguration relative to another, for example comparing two lenses withdifferent base curves but providing the same spherical correction. Imagemagnification depends on the magnitude of the spherical for sphericalequivalent) correction provided, as shown in equation 1.

M=1/[1−t.F ₁ /n]*1/[1−d.F _(v)]  Equation 1

-   Wherein M is the lens magnification;-   t is the lens thickness;-   n is the refractive index of the lens material;-   F₁ is the curvature of the front surface of the lens;-   d is the distance from the back vertex, or distance from the point    of intersection on the lens of the principal axis, to the entrance    pupil of the eye; and-   F_(v) is the back vertex power, or the reciprocal of the distance,    in air, from the back surface of the lens to the secondary focal    point.

Equation 1 shows that image magnification is higher for plus powers andincreases with plus power of a lens.

-   Equation 1 may be rewritten as

SM=1/[1−(t/n)D ₁]*1/[1−d.D],   Equation 2

-   Where SM is the spectacle magnification, defined as the ratio of    retinal image size when wearing a lens of power D to the image size    in an emmetropic eye;-   D₁ is the base curve-   D is the power of the lens in diopters.

Wearers of single vision lenses, in particular myopes who wear minuspower single vision lenses may desire to enhance image magnification fornear objects.

In prior art single vision lenses, it is not possible to alter imagemagnification without changing refractive correction. Since singlevision lens wearers require single vision lenses of a particular powerin order to avoid image blur, they can not be provided a higher level ofimage magnification without causing them to have blurry vision.

Typically, non-presbyopic wearers of all types of vision correction,including users of single vision spectacle lenses or contact lensesexperience reduced contrast sensitivity and visual comfort when viewingnear objects, since near vision tasks typically require finer resolutionand ability to function in low or controlled illumination environments,e.g., indoors at night.

Thus, the object of the present invention is to provide an improvedsingle vision lens whereby image magnification is provided for nearobjects without causing a blurry vision.

This object is solved in accordance with this invention by an improvedsingle lens comprising:

-   -   a continuous change in lens curvature going downwards;    -   a front and a back surfaces;    -   an upper part which front and back surfaces are top zones of        front and back surfaces, covering at least the half of        respectively front and back surfaces, and where the two top        zones are aligned to each other such that the effective power of        the upper part of the lens is constant;    -   a lower part used for viewing near objects which front and back        surfaces are lower zones of front and back surfaces, the front        and back lower zones comprising each a surface zone where top to        bottom increasing curvature of each a surface zone is provided        and where the two front and back lower zones are aligned to each        other such that the maximum difference of the effective power of        the lower part of the lens and the effective power of the upper        part of the lens is comprised between 0 and 0.5 Diopter.

According to the invention an “improved single lens” is a substantiallysingle vision lens having at least the same focal power on the main partof the lens.

According to the invention, “the main part of the lens” corresponds toat least 75% of the surface of the lens.

The topography of the lens curvature may be designed to follow the pathof natural gaze while viewing near objects.

According to an embodiment of the present invention, the maximumdifference of effective power of the lower part of the lens and theeffective power of the upper part of the lens is equal or less to 0.3Diopter.

According to an embodiment of the present invention, front and backsurfaces are aligned to each other such that the effective power of thewhole lens is constant.

According to an embodiment of the present invention, front and backlower zones comprise each an intermediate zone and a bottom zone wherethe curvature of both front and back zones follows a top to downincreasing curvature gradient; a preferred gradient is a lineargradient.

According to the preceding embodiment, the gradient in surface curvatureis less or equal to 3 Diopters.

According to an embodiment of the present invention, the bottom zonesare zones of constant focal power.

According to an embodiment of the present invention, the height h₁ oftop zone, the height h₂ of intermediate zone, the height h₃ of bottomzone of the front surface are respectively equal to the height h₁ of topzone, the height h₂ of intermediate zone, the height h₃ of bottom zoneof the back surface.

According to preceding embodiment, h₁ is comprised between 50 to 70% ofthe total height h of the lens, h₂ is comprised between 5 to 20% of thetotal height h of the lens, h₃ is comprised between 20 to 40% of thetotal height of the lens.

According to another embodiment of the present invention, the front andback surfaces are progressive surfaces comprising each a top far visionzone, an intermediate vision zone and a bottom near vision zone, thesurfaces being adjusted so that the resulting lens power is held to anominal value.

According to preceding embodiment, the front and back progressivesurfaces each comprise a steep curvature zone being disposed along acentral meridian so as the steepening curvature profile matches thecharge in gaze direction as the eyes move from a distant target to anear target.

According to an embodiment of the present invention, the lens isspherical or toric or aspheric.

According to an embodiment of the present invention, the lens is a minuslens for nearsightedness correction, with negative effective power.

According to another embodiment of the present invention, the lens is aplano lens with 0 Diopter effective power.

According to another embodiment of the present invention, the lens is aplus lens for farsightedness correction with positive effective power.

In the frame of the present invention, it has to be understood that aDiopter power value is constant when its variation on a surface is equalor less to 0.1 Diopter.

In the frame of the present invention, it has to be understood that aheight value is equal to another when the difference between the heightsis equal or less to 1 mm.

The present invention is illustrated by way of examples and not limitedby the accompanying figures, in which like references indicate similarelements, and in which:

FIGS. 1 a to c are schematic views of an improved single lens accordingto the present invention.

Skilled artisans appreciate that elements in the figures are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.For example, the dimensions of some of the elements in the figures maybe exaggerated relative to other elements to help improve theunderstanding of the embodiments of the present invention.

According to the present invention, the wordings “top” or “up”, “bottom”or “down” indicate positions in the lens when wearing the lens with anhorizontal gaze, consequently indicating the height of a zone means themaximum distance between two vertical points of said zone when the lensis wear likewise.

The lens 1 of FIG. 1 is an improved single vision lens following anembodiment of the present invention. The lens 1 comprises a frontsurface 2 and a back surface 3.

FIG. 1 a shows a schematic side view of the lens 1, FIG. 1 b shows aschematic front view of the front surface 2 of lens 1 and FIG. 1 c showsa schematic front view of the back surface 3 of lens 1.

The front surface 2 comprises three top to bottom successive zones 21,22, 23 where 21 and 23 are zones of constant curvature and curvature ofbottom zone 23 is higher than curvature of top zone 21 and where thecurvature of intermediate zone 22 is continuously increasing from thecurvature value of top zone 21 at the interface between zones 21 and 22to the curvature value of bottom zone 23 at the interface between zones22 and 23.

The back surface 3 comprises three top to bottom successive zones 31,32, 33 where 31 and 33 are zones of constant curvature and curvature ofbottom zone 33 is higher than curvature of top zone 31 and where thecurvature of intermediate zone 32 is continuously increasing from thecurvature value of top zone 31 at the interface between zones 31 and 32to the curvature value of bottom zone 33 at the interface between zones32 and 33.

The height h₁ of top zone 21 of the front surface 2 is the same as theheight h₁ of the top zone 31 of back surface 3.

The height h₂ of intermediate zone 22 of the front surface 2 is the sameas the height h₂ of intermediate zone 32 of back surface 3.

The height h₃ of bottom zone 23 of the front surface 2 is the same asthe height h₃ of bottom zone 33 of back surface 3.

The total lens height is h, where h=h₁+h₂+h₃For example h=60 mm, h₁=35mm, h₂=7 mm, h₃=18 mm.

Following an embodiment of the present invention, front surface 2 andback surface 3 are aligned to each other, such that the effective powerof the lens as measured by ray tracing analysis is constant everywhere.

Example 1 is an improved single vision lens which has an overallspherical power of −2.0 Diopters, with both surfaces having curvaturesthat vary across the optical surface, as shown in FIGS. 1 b and 1 c. Thetwo surfaces of the lens are aligned to each other, such that theeffective power of the lens as measured by ray tracing analysis is −2.0Diopters everywhere.

Example 2 is an improved single vision lens which has an overall powerof 0.0 Diopter (plano). It is known that even emmetropic persons who donot vision correction may need image magnification in order tocomfortably read fine print or perform tasks that require fineresolution at near distances.

The improved plano single vision lens depicted in example 2 is made upby combining to progressive addition surfaces, one with progressiveaddition power (front surface) and the other with regressive power (backsurface). The power profile of these two surfaces may be adjusted sothat the change in spherical power of the resulting lens is held to avalue below a nominal limit. For example, it is possible to use aprogressive addition surface with a 2.00 Diopters add power and aregressive surface with a 2.00 Diopters power to create the singleimproved vision lens of example 2. In this lens, the front surfacecurvature ranges from 2.00 Diopters to 4.00 Diopters, with therelatively steep zone being disposed along a central meridian, so thatthe steepening curvature profile matches the change in gaze direction asthe eye moves from a distant target to a near target.

The increase in magnification (per cent increase in SM) for such a lens(made of a material of refractive index 1.50) is going from the zonewhere the curvature of the front surface is 2.00 Diopters to the zonewhere the curvature is 4.00 Diopters is 0.5%, ignoring the slightincrease in vertex distance. The impact of base curve steepening onspectacle magnification is more pronounced when the single vision lensprovides a correction. For example, for a plus 4.00 Diopters improvedsingle lens, the front curve might be 6.00 Diopters and the back curve2.00 Diopters. If the near vision zone is steepened by 2.00 Diopters,then the increase in SM is from 1.068 to 1.088, i.e. from 6.8% to 8.8%,or 30%.

This effect is very beneficial for myopes, since the single vision lensproviding refraction corrections to myopes leads to spectacle lensmagnification of less than 1.0 Diopter. This is known in the art asimage minification. For example, a minus lens (−1.50 Diopter) with a 13mm vertex distance and a front curve of 3.5 Diopters made of a materialof refractive index 1.50 is 0.9875, so that the per cent SM is

% SM=−1.25

Example 3 is an improved single vision lens constructed by joining twoprogressive addition surfaces, the front surface with a 3.50 Dioptersbase curve and a 2.00 Diopters add power zone and the back being a 5.00Diopters base curve with a 2.00 Diopters regressive power. The nearvision zone of this lens provides a spherical correction of −1.50Diopters, with a front curve of 5.00 Diopters. The SM at the near visionzone is 0.9905, so that the per cent SM is

% SM=−1.0 (n=1.50)

reducing image minification by 20% compared with the far vision zone.

Therefore, it is proved that image magnification is enhanced or imageminification is reduced in all types of single vision lenses (plus,plano or minus) by increasing the front curve of the lens on thatportion of the lens surface that is used for viewing near objects. Thisapproach of providing a gradient (or an abrupt) increase in lenscurvature at the lower portion of the optic, or that portion of theoptic that is used for viewing near objects will lead to enhanced visualcomfort and better resolution (e.g., contrast sensitivity) whilereading, or performing other tasks that require fine resolution.

The increase in spectacle lens magnification may also be achieved byproviding a small amount of add power (increased sphere power) at thelower portion of the optic, or the near vision zone. The magnitude ofthis increase in spherical correction should be limited so that theresulting defocus or image blurring is not noticeable, or is indeedbelow the level of perception. Typically, an add power 0.10-0.50Diopter, and preferably of 0.10-0.30 Diopter does not cause a change invisual acuity, but provides a small increase in spectacle lens.magnification. For example, an add power of 0.15 Diopter with a frontcurve of 5.00 Diopters in a plano lens provides an SM of 1.012 or a percent SM of 1.2 approximately, while an add power of 0.30 Diopterprovides an SM of 1.014, assuming that the lens material has an index of1.50. Table 1 shows the values of SM for a range of single visionlenses. It is assumed that the material has a refractive index of 1.50,the edge thickness of plus lenses is 1.0 mm and center thickness ofplano and minus lenses is 2.00 mm. Lens thickness were recomputed forthe new front curves in Table 1 which shows the changes in SM with lenspower and front curvature.

TABLE 1 Front Back Spherical Near Add curve, curve, Power, Thickness SM,Prior Curve Power, SM Example Diopter Diopter Diopter mm@OC Art DiopterDiopter invention A 7.50 2.50 +5.00 5.87 1.114 9.50 0 1.126 B 5.00 4.00+1.00 1.95 1.022 7.75 0 1.026 C 6.00 6.00 0.00 2.00 1.008 9.00 0 1.012 D4.00 5.00 −1.00 2.00 0.991 7.00 0 0.995 E 2.00 7.00 −5.00 2.00 0.9334.50 0 0.936 F 5.00 5.00 0.00 2.00 1.007 7.75 0.15 1.013 G 7.00 4.00+3.00 3.95 1.067 8.00 0.15 1.072 H 4.00 4.00 0.00 2.00 1.005 7.00 0.301.014 I 2.00 5.00 −3.00 2.00 0.959 4.75 0.30 0.967

“Prior Art” examples refer to single vision lens where both front andback surfaces have a constant curvature, referred respectively as “frontcurve” and “back curve” in table 1.

The “spherical power” corresponds to resulting effective power of themain part of the lens.

“Thickness @ OC” is the thickness of the lens measured at the opticalcenter of the lens.

“Near curve” is the maximum curvature of the front surface lower zonewith top to bottom increasing curvature, used for lenses according tothe present invention.

The curvature of the back surfaces of Examples A to E, according to theinvention is designed so that the effective power of the whole lens isconstant.

The curvature of the back surfaces of Examples F and G, according to theinvention is designed so that the effective power of the main part ofthe lens is constant and the maximum difference of the effective powerof the lower part of the lens and the effective power of the upper partof the lens is 0.15 Diopter.

The curvature of the back surfaces of Examples H and I, according to theinvention is designed so that the effective power of the main part ofthe lens is constant and the maximum difference of the effective powerof the lower part of the lens and the effective power of the upper partof the lens is 0.30 Diopter.

Table 1 makes possible to compare the spectacle magnification SM ofprior art lenses with lenses according to the present invention.

Table 2 shows the difference in per cent SM values for each of theexamples in Table 1 and the increase in Per Cent SM by changing frontcurvatures of single vision lenses and providing them with moderatevalues of add power.

TABLE 2 Example SM, Prior Art SM invention Diff (% SM) A 1.114 1.126 1.2B 1.022 1.026 0.4 C 1.008 1.012 0.4 D 0.991 0.995 0.4 E 0.933 0.936 0.3F 1.007 1.013 0.6 G 1.067 1.072 0.5 H 1.005 1.014 0.9 I 0.959 0.967 0.8

It is known that the wearers of spectacle lenses can visually perceive abinocular difference of 0.75% to 1% in per cent SM. It is expected thata difference of 0.5% or more is preferably chosen to provide asignificant improvement in visual function. This lower limit of efficacywill correspond to either an increase of 2.50 Diopters or more in thefront curve is preferred for plano or minus lenses, or an add power tobe provided at the level of 0.15 Diopters to 0.30 Diopters. Therequirements are less stringent for plus lenses, which show significantimprovement in per cent SM when the front curve is steepened by 2.00Diopters with no provision for add power. The most effective combinationis to provide both means to increase per cent SM. Maintaining the addpower to a value less than or equal to 0.30 Diopter will ensure that theunwanted astigmatism that develops as a consequence of providing the addpower will be less or equal to 0.25 Diopter, and may be pushed outwardwhere it can not interfere with direct gaze in any eye position. Thesteepening of the front curve (without providing an add power) will alsointroduce surface astigmatism on the front surface. The effect of thissurface astigmatism will be apparent in causing image distortion, unlessit is compensated by providing a complementary curve on the backsurface. In single vision lenses with net spherical power, it is notpossible to provide a complete neutralization of the added sphericalpower as well as astigmatism. A high quality design of the overall lensmay be derived by performing a simultaneous optimization of bothsurfaces using an algorithm that compute retinal image distortion byperforming ray tracing analysis through multiple points of the lensoptic, with the lens positioned in an “as worn” position.

A method for defining a lens by optimizing the optical characteristicsof the lens is disclosed in patent document U.S. Pat. No. 6,318,859 toT. Baudart et al. which is enclosed by reference, where opticalcharacteristics are calculated during optimization using a ray tracingprogram, under wearing conditions. Said method is suitable to perform asimultaneous optimization of both surfaces according to the presentinvention.

Another method suitable to perform a simultaneous optimisation of bothsurfaces according to the invention is disclosed in followingpublication: “Applications of optimization in computer-aided ophthalmiclens design”—P. Allione, F. Ahsbahs and G. Lessaux—SPIE Vol. 3737—p.138-148 (May 1999).

Still another suitable method is disclosed in patent applicationPCT/IB2006/003220.

In addition, those of ordinary skill in the art will perceive thatchanging vertex distance or changing pantoscopic tilts may also providesome relief when needed.

This concept may be applied to both spherical and toric single visionlenses. The value of the spectacle lens magnification is preferablycomputed in every meridian, since the power (effective power orspherocylindrical power) will be different at each meridian. Theincrease in SM between state of the art designs and designs disclosed inthis invention will be proportionately the same as the front curve isaltered or when moderate add power is provided.

In addition to other design elements described above the surfaces of thesingle vision optic may be further enhanced by rendering them aspheric.Advantages of using aspheric surfaces include a wider field of view, andreduced thickness of plus lenses.

The fabrication of these novel single vision lenses requires the use ofstate of the art lens machining or molding technology. The steepening ofthe front curve of the increase in spherical power is preferablyprovided at the inferior portion of the optic. The optic design isrelatively insensitive to the precise orientation of the centralmeridian of the altered zone. Therefore, it may be possible to cover allpossible values of the axis of the prescribed cylinder with two possibleorientations of the altered zones, such as inferior nasal and inferiortemporal. In some case, the front curve will have a highly complex, nonsymmetric form, and is preferably provided by molding or casting a blankagainst an optical tool embodying this geometry, or using a free formmachining process. Every spectacle lens material may be used in theframe of the present invention such as non limiting materials knownunder commercial references <<ORMA>>, <<PC>>, <<MR8>>, <<MR7>>,<<1.74>>.

The invention has been described above with the aid of embodimentswithout limitation of the general inventive concept which is evidentfrom the claims and the general portion of the specification.

1. An improved single lens comprising: a continuous change in lenscurvature going downwards; a front and a back surfaces; an upper partwhich front and back surfaces are top zones of front and back surfaces,covering at least the half of respectively front and back surfaces, andwhere the two top zones are aligned to each other such that theeffective power of the upper part of the lens is constant; a lower partused for viewing near objects which front and back surfaces are lowerzones of front and back surfaces, the front and back lower zonescomprising each a surface zone where top to bottom increasing curvatureof each surface zone is provided and where the two front and back lowerzones are aligned to each other such that the maximum difference of theeffective power of the lower part of the lens and the effective power ofthe upper part of the lens is comprised between 0 and 0.5 Diopter. 2.The improved single lens of claim 1 where the maximum difference ofeffective power of the lower part of the lens and the effective power ofthe upper part of the lens is equal or less to 0.3 Diopter.
 3. Theimproved single lens of claim 1 where front and back surfaces arealigned to each other such that the effective power of the whole lens isconstant.
 4. The improved single lens according to claim 1 where thefront and back lower zones comprise each an intermediate zone and abottom zone, where the curvature of both front and back zones follows atop to down increasing curvature gradient.
 5. The improved single lensaccording to claim 4 where the gradient in surface curvature is less orequal to 3 Diopters.
 6. The improved single lens according to claim 4where the bottom zones are zones of constant focal power.
 7. Theimproved single lens according to claim 4 where the height h₁ of topzone, the height h₂ of intermediate zone, the height h₃ of bottom zoneof the front surface are respectively equal to the height h₁ of topzone, the height h₂ of intermediate zone, the height h₃ of bottom zoneof the back surface.
 8. The improved single lens according to claim 7where h₁ is comprised between 50 to 70% of the total height h of thelens, h₂ is comprised between 5 to 20% of the total height h of thelens, h₃ is comprised between 20 to 40% of the total height of the lens.9. The improved single lens according to claim 1 where the front andback surfaces are progressive surfaces comprising each a top far visionzone, an intermediate vision zone and a bottom near vision zone, thesurfaces being adjusted so that the resulting lens power is held to anominal value.
 10. The improved single lens according to claim 9 wherethe front and back progressive surfaces each comprise a steep curvaturezone being disposed along a central meridian so as the steepeningcurvature profile matches the charge in gaze direction as the eyes movefrom a distant target to a near target.
 11. The improved single lensaccording to claim 1 where the lens is spherical or tonic or aspheric.12. The improved single lens according to claim 1 where the lens is aminus lens for nearsightedness correction with negative effective power.13. The improved single lens according to claim 1 where the lens is aplano lens with 0 Diopter effective power.
 14. The improved single lensaccording to claim 1 where the lens is a plus lens for farsightednesscorrection with positive effective power.